Nickel base alloy

ABSTRACT

A nickel base alloy comprising: (measured in % by weight): 11-16% Co; 12.2-15.5% Cr; 6.5-7.2% Al; 3.2-5.0% Re; 1.0-2.5% Si; 1.5-4.5% Ta; 0.2-2.0% Nb; 0.2-1.2% Hf; 0.2-1.2% Y; 0-1.5% Mg; 0-1.5% Zr; 0-0.5% La and La series elements; 0-0.15% C; 0-0.1% B; and a remainder including Ni and impurities. The alloy is particularly suited for coatings for gas turbine components such as gas turbine blades and vanes.

TECHNICAL FIELD

The invention relates to a nickel base alloy.

BACKGROUND OF THE INVENTION

This invention relates to nickel-based alloys, especially for those usedas a coating for high temperature gas turbine blades and vanes.

Wide use of single crystal (SX) and directionally solidified (DS)components has allowed increased turbine inlet temperature and thereforeturbine efficiency. Alloys, specially designed for SX/DS casting, weredeveloped in order to make a maximum use of material strength andtemperature capability. For this purpose modem SX alloys contain Ni andsolid-solution strengtheners such as Re, W, Mo, Co, Cr as well asγ′-forming elements Al, Ta, Ti. The amount of refractory elements in thematrix has continuously increased with increase in the required metaltemperature. In a typical SX alloys their content is limited byprecipitation of deleterious Re-, W-or Cr-rich phases.

High temperature components are typically coated to protect them fromoxidation and corrosion. In order to take full advantage of increasedtemperature capability and mechanical properties of SX/DS blade basematerial, coating material must provide now not only protection fromoxidation and corrosion, but must also not degrade mechanical propertiesof base material and have a stable bond to substrate without spallationduring the service. Therefore requirements for advance coatings are:

high oxidation and corrosion resistance, superior to those of the SX/DSsuperalloys;

low interdiffusion of Al and Cr into the substrate to preventprecipitation of needle-like phases under the coating;

creep resistance comparable to those of conventional superalloys, whichcan be achieved only with the similar coherent γ-γ′ structure;

low ductile-brittle transition temperature, ductility at lowtemperature;

thermal expansion similar to substrate along the whole temperaturerange.

The coating described in U.S. Pat. No. 5,043,138 is a derivative of thetypical SX superalloy with additions of yttrium and silicon in order toincrease oxidation resistance. Such coatings have very high creepresistance, low ductile-brittle transition temperatures (DBTT), thermalexpansion coefficients equal to those of the substrate and almost nointerdiffusion between coating and substrate. However, the presence ofsuch strengtheners as W and Mo, as well as a low chromium and cobaltcontent typical for the SX superalloys, has a deleterious effect onoxidation resistance. European Patent Publication 0 412 397 A1 describesa coating with significant additions of Re, which simultaneouslyimproves creep and oxidation resistance at high temperature. However,the combination of Re with a high Cr content, typical for traditionalcoatings, results in an undesirable phase structure of the coating andinterdiffusion layer. At intermediate temperatures (below 950-900° C.),the α-Cr phase is more stable in the coating than the γ-matrix. Thisresults in a lower thermal expansion compared to the base material, alower toughness and possibly a lower ductility. In addition, asignificant excess of Cr in the coating compared to the substrateresults in diffusion of Cr to the base alloy, which makes it prone toprecipitation of needle like Cr-, W- and Re-rich phases.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention is to provide an nickel basealloy which is designed to combine an improved ductility and creepresistance, phase stability of coating and substrate during service,phase structure and thermal expansion similar to the substrate and anexcellent oxidation resistance.

The invention provides a nickel base alloy, particularly useful as acoating, which comprises: (measured in % by weight):

Co 11-16 Cr 12.2-15.5 Al 6.5-7.2 Re 3.2-5.0 Si 1.0-2.5 Ta 1.5-4.5 Nb0.2-2.0 Hf 0.2-1.2 Y 0.2-1.2 Mg   0-1.5 Zr   0-1.5 La and La-serieselements   0-0.5 C   0-0.15 B   0-0.1 a remainder including Ni andimpurities

The advantages of the invention can be seen, inter alia, in the factthat by optimisation of Al activity in the alloy and due to the-specificphase structure, consisting of fine precipitates of γ′ and α-Cr inγ-matrix an improved ductility and creep resistance, phase stability ofcoating and substrate during service, phase structure and thermalexpansion similar to the substrate and an excellent oxidation resistancecan be obtained. To achieve the γ-γ′-α-Cr-structure the relatively highbut limited contents of Al and Cr were combined. To prevent coarseningof the α-Cr phase an addition of more than 3% Re was necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and the attendantadvantages thereof will be readily obtained by reference to theaccompanying drawings, wherein:

FIG. 1 shows Al activity vs. Al content in a γ-γ′-α-Cr system;

FIG. 2 shows Al activity vs. Cr content in a γ-γ′-α-Cr system;

FIG. 3 shows Al activity vs. Si content in a γ-γ′-α-Cr system;

FIG. 4 shows Al activity vs. Re content in a γ-γ′-α-Cr system;

FIG. 5 shows the phase structure of the LSV-1 coating with fineprecipitates of α-Cr, Re phase which is white due to high Re content andedge effect;

FIG. 6 shows the phase structure of the LSV-6 coating with undesirablechain-like distributions of β-(black) and σ-(gray) phases; and

FIG. 7 shows the phase structure of the LSV-5 coating with coarsepentagonal precipitates of α-Cr phase.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention describes a nickel base superalloy, whose essentialcomposition range is shown in Table 2, which is particularly adapted foruse as a coating for advanced gas turbines blades and vanes. Generally,Table 1 shows the alloys as used during the experiments. From theexperimental coatings only LSV 3 is an alloy which has a compositionaccording to the invention. Preferably, the alloy could be produced bythe vacuum melt process in which powder particles are formed by inertgas atomisation. The powder can then be deposited on a substrate using,for example, thermal spray methods. However, other methods ofapplication may also be used. Heat treatment of the coating usingappropriate times and temperatures is recommended to achieve a good bondto the substrate and a high sintered density of the coating. The alloychemical composition is specifically designed to combine an improvedductility and creep resistance, phase stability of the coating andsubstrate during service, phase structure and thermal expansion similarto the substrate and an excellent oxidation resistance due to highactivity of Al. This is achieved by optimisation of Al activity in thealloy (FIGS. 1-4) and due to the specific phase structure, consisting offine precipitates of γ′ (55-65 vol.%) and α-Cr (1.5-3 vol.%) in γ-matrix(alloys LSV 1,3, FIG. 5). To achieve this structure the relatively highcontents of Al (about 7%) and Cr (about 13%) were combined. To preventcoarsening of the α-Cr phase an addition of more than 3% Re wasnecessary. The composition of experimental coatings are shown inTable 1. Table 3 represents results of experimental evaluation ofseveral compositions of coatings with respect of their oxidationresistance and mechanical properties. Upon oxidation the alloy shows anincrease in weight due to the uptake of oxygen. If the growing oxidescale is protective the weight gain as a function of oxidation timefollows a parabolic rate law. Obviously, a small weight increase isindicative of a slowly growing oxide scale and, thus, is a desirableproperty. Presented in Table 3 are experimental data which show that theweight change is lowest for the preferred alloy composition (LSV 1,3)when compared to experimental alloys LSV 4,5,7,10,11. The oxidationresistance of the inventive alloy is determined by Al content (asreservoir of Al atoms for formation of protective Al₂O₃ scale) byactivity of Al in the system, by alloy phase structure, which determinesAl diffusion and by control over oxide growth rate through controlledaddition of active elements, i.e combination of Ta and Nb. Presence andcontent of other elements has a very strong effect on the activity ofAl. Examples modelled for γ-γ′-α-Cr system using known computer software(ThermoCalc and DICTRA), are presented on FIGS. 1-4 (for varied Al, Cr,Si and Re respectively with fixed content of other elements, referencesystem Ni-13 Cr-12 Co-7 Al-3.5 Re-2 Si-3 Ta-1 Nb).

FIG. 1 shows, that for the Al content higher than 6.5%, activity of Al(and therefore the oxidation resistance of the alloy) increases mostefficiently. This is illustrated by comparison of properties of alloysLSV-1 and LSV-10 (Table 3). Their chemical composition is identical withexception of the Al level (7% and 6.1% respectively).

If Al content exceeds some particular level (7.2% in the presentsystem), the precipitation of β-and σ-phases with undesirable morphologyreduces the low temperature ductility of alloys (alloy LSV-6, FIG. 6,Table 3,4).

Very tight control is also required for the Cr content. The low Crcontent results not only in low corrosion resistance of the coating, butalso in lower activity of Al and therefore considerably lower oxidationresistance. This is illustrated in FIG. 2, which shows, that the highestactivity of Al in the alloy can be achieved at Cr contents higher than12%. Below this level the Al₂O₃ scale is not dense and additional Ni andCr oxides reduces the oxidation resistance. Comparison of properties ofalloys LSV 1, 3 and alloy LSV-11 from Table 3 shows this effect on theother hand, Cr contents higher then 15.5%, result in significantreductions in low temperature ductility of the alloy (alloy LSV-9, Table1,3,4). At this concentration of Cr and other elements, the morethermodynamically stable at intermediate (below 900° C.) temperaturesα-Cr phase replaces to a large extent the ductile γ-matrix during theservice exposure, which results in considerable enbrittlement of thecoating. Resulting α-Cr-σ-γ′-γ or α-Cr-β-γ′-γ structures are much lessductile than the γ-γ′ structure with fine α-Cr precipitates chosen forthe coatings of the present invention.

Co increases the solubility of Al in the γ-matrix. The relatively highCo level in alloys of the present invention allows the achievement ofuniquely high concentrations of both Al and Cr in the γ-matrix withoutprecipitation of the aforementioned undesirable β- and σ- phases, andtherefore allows for increased oxidation resistance of the alloy withouta reduction in mechanical properties. A comparison of the properties ofLSV-1 and LSV-3 with those of the alloy LSV-4, which is similar to thecompositions of U.S. Pat. No. 5,035,958, confirms the beneficial role ofa high Co content (Table 3). A high level of Co (more than 16%) resultsin a significant lowering of the γ′-solvus temperature compared to thebase alloy. Therefore, at temperatures above the coating γ′-solvus andbelow the substrate γ′-solvus, the two materials have a high thermalexpansion mismatch which leads to a significant reduction in the coatingthermomechanical-fatigue-(TMF)-life.

Re in the alloy replaces other refractory elements such as W and Mo andprovides high creep and fatigue resistance to the coating withoutdeleterious effect on oxidation and corrosion resistance. Moreover, Reincreases the activity of Al in the alloy and therefore is beneficialfor oxidation resistance (FIG. 4). At same time Re is responsible forstabilising the fine morphology of γ′ particles which also considerablyimproves creep properties. These functions of Re are relatively linearto its content in the alloy and are known from the art. What was foundnew in the present invention, is that in the γ-γ′-α structure Reconsiderably changes α-Cr composition and morphology, but only aftersome particular level in the alloy. At contents up to 3%, Repartitioning occurs mostly in the γ-matrix, similar to it's behaviour insuperalloys. The α-Cr phase at low Re concentrations consists of 95 at.% of Cr with 1-2 at.% of each Ni, Re, Co. The α-Cr precipitates havecoarse pentagonal morphology with sizes on the order of 3-6 μm (as inalloy LSV-5, FIG. 7). The excess of Re and Cr in the matrix precipitatesseparately in the undesirable form of needle-like Re-rich TCP phases (socalled r- and p-phases), especially at the interface with the substrate,and mechanical properties of the system are reduced to see (Table 3,alloy LSV 5 compared to alloys LSV 1, 3). At the Re contents higher than3%, the type of α-phase changes from a Cr phase to a mixed Cr-Re phase(with 15-20 at. % of Re and up to 8 at. % of Co, Table 4,5). The newphase has much finer morphology (size is 1 μm and smaller) and itspresence prevents also precipitation of needle-like Re-rich r- andp-phases, since the solubility range of Re and Co in the α-Cr-Re phaseis relatively wide. The condition, where the desirable Cr-Re α-phaseprecipitates is described (for Al range 6.5-7.2% and in presence of Ta,Nb, Si; W+Mo=0; Re>3%) as

(Re+0.2Co)/0.5Cr=0.9,  {1}

where Re, Co, Cr are the contents of elements in the alloy in wt. %. At(Re+0.2 Co)/0.5 Cr<0.9 the coarse α-Cr and needle-like Re-rich TCPphases precipitate.

Typically, MCrAlY coatings contain 0.3 to 1 wt % Y which has a powerfuleffect on the oxidation resistance of the alloy. In some fashion, Y actsto improve the adherence of the oxide scale which forms on the coating,thereby substantially reducing spallation. A variety of other so-calledoxygen active elements (La, Ce, Zr, Hf, Si) have been proposed toreplace or supplement the Y content. Patents which relate to the conceptof oxygen active elements in overlay coatings include U.S. Pat. Nos.4,419,416 and 4,086,391. In the present invention Y is added in amountson the order of 0.3 to 1.3 wt %, La and elements from the Lanthanideseries in amounts ranging from 0 to 0.5 wt %. In the present inventionNb and Ta were found to increase oxidation resistance through reducingthe rate of oxide growth, with their cumulative effect stronger than theinfluence of any one of them taken separately. Even small amounts of Nbon the order of 0.2-0.5 wt % in the presence of Ta has found to have asignificant effect on oxidation resistance (preferred compositionresults vs. LSV-7, Table 3).

Si in the alloy increases oxidation resistance by increasing theactivity of Al (FIG. 4). The Si effect on Al activity becomessignificant first at a Si content higher than 1%. At the same time, theSi content higher than 2.5% results in precipitation of brittle Ni (Ta,Si) Heusler phases and in embrittlement of a γ-matrix.

The range of composition for Hf, Y, Mg, Zr, La, C and B is optimized foroxidation lifetime of the coating.

The invention is of course not restricted to the exemplary embodimentshown and described.

TABLE 1 Composition of experimental coatings Coating Ni Co Cr Al Y Hf ReSi Ta Nb LSV-1 bal 12 12.5 7 0.3 — 3.5 1.2 1.5 0.3 LSV-3 bal 12 15 7 0.30.3 4.5 2.1 3 0.5 LSV-4* bal 10 11 7 0.3 0.3 3.2 2.1 3 0.5 LSV-5 bal 1213 7 0.3 0.3 2.8 2.1 3 0.5 LSV-6 bal 12 15 7.7 0.3 0.3 4.5 2.1 3 0.5LSV-7 bal 12 13 7 0.3 0.3 3.5 1.2 2.1 — LSV-9 bal 12 20 6.7 0.5 0.3 3.51.2 3 0.5 LSV-10 bal 12 12.5 6.1 0.3 — 3.5 1.2 1.5 0.3 LSV-11 bal 12 8.57 0.5 0.5 3.0 2 3 0.3 LSV-4*: W = 2.5 wt. %, Mo = 1 wt. %

TABLE 2 Preferred range of the alloy according to the invention CoatingNi Co Cr Al Hf Re Si Ta Nb SV16 bal 11-16 12.5-15.5 6.5-7.2 0.2-1.23.2-5 1-2.5 1.5-4.5 0.2-2 Coating Y Mg Zr La* C B Y + Zr + La* (Re +0.2Co)/0.5Cr SV16 0.2-1.2 0-1.5 0-1.5 0-0.5 0-0.15 0-0.1 0.3-2.0 0.9-1.2La* = La and La-series elements

TABLE 3 Experimental evaluation of coatings Ductility after ageing atOxidation resistance at 900° C. Elongation of 1000° C. Weight gaincoated tensile specimen after 1000 h of isothermal (CMSX-4) at themoment Coating oxidation test, mg/cm² of coating failure, RT/400° C.; %;LSV-1 1.0 >10/>10 LSV-3 0.8 >10/>10 LSV-4 5.8 >10/>10 LSV-5 3.0 3.2/7.0LSV-6 0.8 2.3/3.6 LSV-7 3.9 >10/>10 LSV-9 1.0 2.5/5.0 LSV-10 4.5 >10/>10LSV-11 7.2 >10/>10

TABLE 4 Phase volume fraction in structure of experimental coatings,vol. % Coating γ γ′ β σ,r α-Cr,Re α-Cr LSV-1 36 62 2 LSV-5 19 70 6 5LSV-6 36 41 18 5 LSV-9 27 55 4 14

TABLE 5 Phase composition of α phase in experimental coatings, at. %Coating Phase Ni Co Cr Re Si LSV-5 α-Cr 2 2 91 3 2 LSV-1 α-Cr,Re 1 5 7518 1

What is claimed is:
 1. A nickel base alloy, comprising: (measured in %by weight): Co 11-16; Cr 12.2-15.5; Al 6.5-7.2; Re 3.2-5.0; Si 1.0-2.5;Ta 1.5-4.5; Nb 0.2-2.0; Hf 0.2-1.2; Y 0.2-1.2; Mg 0-1.5; Zr 0-1.5; Laand La-series elements 0-0.5; C 0-0.15; B 0-0.1; and a remainderincluding Ni with impurities wherein (Re+0.2 Co)/0.5 Cr is not less than0.9 and Y+Zr+ (La+La-series) ranges from 0.3-2.0.
 2. A nickel base alloyas claimed in claim 1, having a phase structure consisting of fineprecipitates of γ′ and α-Cr in a γ-matrix.
 3. A coating comprised of thenickel base alloy as claimed in claim 1, having a phase structureconsisting of fine precipitates of γ′ and α-Cr in a γ-matrix.
 4. Acoating as claimed in claim 3, wherein the fine precipitates of γ′ranges from 55 to 65 vol. % and the α-Cr ranges from 1.5 to 3 vol. % inthe γ-matrix.
 5. A nickel base alloy as claimed in claim 1, comprising acoating for gas turbine components.
 6. A nickel base alloy as claimed inclaim 1, comprising a coating for gas turbine blades and vanes.
 7. Anickel base alloy as claimed in claim 1, consisting essentially ofmeasured in % by weight Co 11-16; Cr 12.2-15.5; Al 6.5-7.2; Re 3.2-5.0;Si 1.0-2.5; Ta 1.5-4.5; Nb 0.2-2.0; Hf 0.2-1.2; Y 0.2-1.2; Mg 0-1.5; Zr0-1.5; La and La-series elements 0-0.5; C 0-0.15; B 0-0.1; and aremainder including Ni with impurities wherein (Re+0.2 Co)/0.5 Cr is notless than 0.9 and Y+Zr+ (La+La-series) ranges from 0.3-2.0.
 8. A nickelbase alloy as claimed in claim 1, comprising a thermally sprayed coatingon a turbine blade or turbine vane.
 9. A nickel base alloy as claimed inclaim 1, wherein Cr and Re form a mixed α-Cr-Re phase with 15 to 20atomic % Re and up to 8% Co, the mixed αCr-Re phase having a size of 1μm and smaller.
 10. A nickel base alloy as claimed in claim 1, whereinthe alloy is W-free.
 11. A nickel base alloy as claimed in claim 1,wherein the Nb content is 0.2 to 0.5%.
 12. A nickel base alloy asclaimed in claim 1, wherein the alloy is Mo-free.